Facile Synthesis of 2D/2D Ti2C3/ZnIn2S4 Heterostructure for Enhanced Photocatalytic Hydrogen Generation

ZnIn2S4, a novel two-dimensional visible light-responsive photocatalyst, has attracted much attention in the photocatalytic evolution of H2 under visible light irradiation due to its attractive intrinsic photoelectric properties and geometric configuration. However, ZnIn2S4 still has severe charge recombination, which results in moderate photocatalytic performance. Herein, we report the successful synthesis of 2D/2D ZnIn2S4/Ti3C2 nanocomposites by a facile one-step hydrothermal method. The efficiency of the nanocomposites in photocatalytic hydrogen evolution under visible light irradiation was also evaluated for different ratios of Ti3C2, and the optimal photocatalytic activity was achieved at 5% Ti3C2. Importantly, the activity was significantly higher than that of pure ZnIn2S4, ZnIn2S4/Pt, and ZnIn2S4/graphene. The enhanced photocatalytic activity is mainly due to the close interfacial contact between Ti3C2 and ZnIn2S4 nanosheets, which amplifies the transport of photogenerated electrons and enhances the separation of photogenerated carriers. This research describes a novel approach for the synthesis of 2D MXenes for photocatalytic hydrogen production and expands the utility of MXene composite materials in the fields of energy storage and conversion.


Introduction
As a kind of sustainable clean energy, hydrogen energy has great application potential in solving the problem of energy shortage. Fujishima and Honda were the first to split water into H 2 and O 2 via a titania (TiO 2 ) electrode in 1972 [1]. Since then, semiconductor photocatalytic hydrogen production has been purposefully pursued due to its potential applications in the production of clean hydrogen energy [2,3]. However, the application of photocatalysis is limited by the absorption of sunlight, the efficiency of charge separation, and the interfacial catalytic reaction [4].
One strategy to improve the photocatalytic activity is through the development and use of two-dimensional (2D) nanocrystallization of semiconductor photocatalytic materials. These materials offer larger specific surface area with more active sites, while the ultra-thin nano layer can shorten the carrier transport distance and facilitate its migration to the catalyst surface. ZnIn 2 S 4 is a promising photocatalytic H 2 generation photocatalyst due to its desirable band gap in the visible light region [5,6]. In general, ZnIn 2 S 4 shows two different crystal phases of hexagonal lattice and cubic lattice. A large number of previous reports indicated that hexagonal phases had better stability and higher photocatalytic activity. Most often, flake structures have been observed in the morphology of hexagonal ZnIn 2 S 4 due to the stacking of S-Zn-S-In-S-In-S layers [7][8][9][10]. Advantageously, the unique structure of 2D ZnIn 2 S 4 nanosheets decreases the diffusion distance of electrons and improves the separation of photoinduced charges, while a large number of exposed surface atoms can provide rich active sites [11].
However, due to the low separation efficiency and migration capacity of photoexcited charge carriers, the photocatalytic activity of pure ZnIn 2 S 4 is insufficient [12,13]. Over the 2 of 13 years, numerous efforts have been made to improve ZnIn 2 S 4 , such as doping with metal ions [14,15], combination with semiconductor [16], construction of heterojunction [17], coupling cocatalyst [18], etc. The construction of 2D/2D heterostructures can form more charge transfer channels, which is conducive to the spatial separation and migration of carriers [19,20]. Therefore, the assembly of new heterostructures is considered an effective strategy to improve the photocatalytic performance of ZnIn 2 S 4 .
Ti 3 C 2 MXene, as a new 2D material, has attracted extensive attention in photocatalysis because of its special metallic conductivity. Ti 3 C 2 is rich in hydrophilic functional groups (-OH, -O, -F) on the surface, which is favorable for composites with semiconductor photocatalytic materials. A sufficiently positive Fermi level (0.71 eV vs. NHE) of Ti 3 C 2 is conducive to the transfer of photoelectrons from semiconductors to Ti 3 C 2 , as well as the effective separation of electron-hole pairs. Recently, it has been reported that ZnIn 2 S 4 nanosheets were grown in situ on Ti 3 C 2 nanosheets to form a sandwich-like hierarchical heterostructure [21]. The unique hierarchical structure confers the system with superior photoexcited charge transport ability, which further accelerates photoexcited charge separation.
In this study, 2D/2D ZnIn 2 S 4 /Ti 3 C 2 nanocomposites were successfully prepared by a simple one-step hydrothermal method, and the photocatalytic performance of the composites with different ratios of Ti 3 C 2 was evaluated in the visible light region. The photocatalytic activity reached the maximum for a mass ratio of 5% Ti 3 C 2 , and it was significantly higher than that of pure ZnIn 2 S 4 , ZnIn 2 S 4 /Pt, and ZnIn 2 S 4 /graphene. The enhanced photocatalytic activity is mainly due to the close interfacial contact between Ti 3 C 2 and ZnIn 2 S 4 nanosheets, which accelerates the transport of photoelectrons and increases the separation of photogenerated carriers.

Characterizations
XRD was used to determine the crystalline phases of the samples, as shown in Figure 1. Notably, the largest diffraction peak for the (104) plane of Ti 3 AlC 2 , which was at 39.0 • , disappeared after HF treatment, indicating the removal of Al layers of Ti 3 AlC 2 [22]. In addition, the diffraction peaks of the (002) plane at 9.4 • and the (004) plane at 19.1 • shifted to lower degrees and become broader, which was ascribed to increase in the interlayer spacing of Ti 3 C 2 [23]. This can also be evidenced by SEM images of Ti 3 C 2 (Figure 2b). Compared with Ti 3 AlC 2 (Figure 2a), Ti 3 C 2 has an obviously layered microstructure. These results confirmed the successful transformation of Ti 3 AlC 2 into layered Ti 3 C 2 .     SEM and TEM were used to observe the morphologies of the samples. The pure ZnIn2S4 had a 3D hierarchical flower-like structure self-assembled by 2D nanosheets (Figure 4) according to previous reports [24,25]. The SEM images of ZnIn2S4/Ti3C2 composites (ZIST-5) showed ultrathin nanosheets of ZnIn2S4 densely populating the surface of the layered Ti3C2 (Figure 5a). It can be clearly observed that ZnIn2S4 is an outer layer closely attached to the of surface Ti3C2, which is the inner layer in Figure 5b. Ti3C2 retains an integrated lamellar structure in the TEM images (Figure 5c) of ZnIn2S4/Ti3C2. Lattice spacing of 0.32 nm and lattice fringes of 0.26 nm were determined in the HRTEM image of ZnIn2S4/Ti3C2, corresponding to the (102) plane of hexagonal ZnIn2S4 [18] and the (010) plane of Ti3C2, respectively [26]. These results indicate that ZnIn2S4 nanosheets were anchored on Ti3C2 rather than simply mixed. ZnIn 2 S 4 /Ti 3 C 2 nanocomposites were synthesized via a facile one-step hydrothermal method. The mass ratio of ZnIn 2 S 4 to Ti 3 C 2 was varied to determine the optimal composition, and mass ratios of 2 wt.%, 5 wt.%, 10 wt.%, 20 wt.%, and 50 wt.% corresponded to samples ZIST-2, ZIST-5, ZIST-10, ZIST-20, and ZIST-50, respectively. Figure 3 presents the XRD patterns of ZnIn 2 S 4 /Ti 3 C 2 composites with different Ti 3 C 2 content, and the ZnIn 2 S 4 /Ti 3 C 2 composites had similar diffraction peaks to that of hexagonal ZnIn 2 S 4 [18]. In addition, no obvious diffraction peaks of Ti 3 C 2 were found in the XRD patterns of the composites, indicating the limited amount and weak diffraction intensity of Ti 3 C 2 . Meanwhile, no diffraction peak of TiO 2 was found, which suggested Ti 3 C 2 will not transform into TiO 2 at 150 • C. Therefore, the temperature for sample synthesis is feasible in this work.   SEM and TEM were used to observe the morphologies of the samples. The pure ZnIn 2 S 4 had a 3D hierarchical flower-like structure self-assembled by 2D nanosheets (Figure 4) according to previous reports [24,25]. The SEM images of ZnIn 2 S 4 /Ti 3 C 2 composites (ZIST-5) showed ultrathin nanosheets of ZnIn 2 S 4 densely populating the surface of the layered Ti 3 C 2 ( Figure 5a). It can be clearly observed that ZnIn 2 S 4 is an outer layer closely attached to the of surface Ti 3 C 2 , which is the inner layer in Figure 5b. Ti 3 C 2 retains an integrated lamellar structure in the TEM images (Figure 5c) of ZnIn 2 S 4 /Ti 3 C 2 . Lattice spacing of 0.32 nm and lattice fringes of 0.26 nm were determined in the HRTEM image of ZnIn 2 S 4 /Ti 3 C 2 , corresponding to the (102) plane of hexagonal ZnIn 2 S 4 [18] and the (010) plane of Ti 3 C 2 , respectively [26]. These results indicate that ZnIn 2 S 4 nanosheets were anchored on Ti 3 C 2 rather than simply mixed.   Next, the surface chemistry of the photocatalyst was investigated by XPS. As shown in Figure 6, a weak signal was observed in the Ti 2p spectrum of ZIST-5, which is mainly due to the existence of Ti3C2 in the inner layer of the sample and its content is too low. With the increase in Ti3C2 content, the Ti 2p signal becomes stronger. When the mass ratio of Ti3C2 reaches 50% of ZnIn2S4 (ZIST-50), four peaks at 454.9, 459.2, 461.8, and 464.8 eV Next, the surface chemistry of the photocatalyst was investigated by XPS. As shown in Figure 6, a weak signal was observed in the Ti 2p spectrum of ZIST-5, which is mainly due to the existence of Ti 3 C 2 in the inner layer of the sample and its content is too low. With the increase in Ti 3 C 2 content, the Ti 2p signal becomes stronger. When the mass ratio of Ti 3 C 2 reaches 50% of ZnIn 2 S 4 (ZIST-50), four peaks at 454.9, 459.2, 461.8, and 464.8 eV were observed in the Ti 2p spectrum, correlating with Ti-O 2p1/2, Ti-C 2p1/2, Ti-O 2p3/2, and Ti-C 2p3/2, respectively [27]. Ti-O is unlikely to be attributable to TiO 2 , because it can be seen from XRD that Ti 3 C 2 will not transform into TiO 2 under the reaction conditions of preparing ZnIn 2 S 4 /Ti 3 C 2 . Ti-O is formed during the preparation of Ti 3 C 2 due to the substitution of O or OH for Tx in Ti 3 C 2 Tx [28]. Consistent with the Ti 2p spectrum, the C1s peak at 281.6 eV was assigned to Ti-C, and its intensity increased with increasing Ti 3 C 2 content ( Figure 7). Moreover, compared with pure ZnIn 2 S 4 , the Zn 2p and In 3d peaks in the XPS spectra of ZnIn 2 S 4 /Ti 3 C 2 shifted towards higher binding energies, while the S 2p peak shifted to lower binding energy. These changes suggest a strong electronic interaction between Ti 3 C 2 and ZnIn 2 S 4 (Figures 8-10).   DRS was used to measure the light harvesting capability of the photocatalyst samples. A clear red shift was exhibited for the ZnIn2S4/Ti3C2 nanocomposites, and their absorption in the visible light region was significantly enhanced compared with pure ZnIn2S4 ( Figure 11). As the Ti3C2 content increased, the absorption intensity of ZnIn2S4/Ti3C2 gradually increased due to the full-spectrum absorption of dark Ti3C2 [23]. Because of the high efficiency of photothermal conversion of Ti3C2 [22], it is possible to activate the catalyst by converting the light energy into heat to promote the surface catalytic reaction [29]. DRS was used to measure the light harvesting capability of the photocatalyst samples. A clear red shift was exhibited for the ZnIn2S4/Ti3C2 nanocomposites, and their absorption in the visible light region was significantly enhanced compared with pure ZnIn2S4 ( Figure 11). As the Ti3C2 content increased, the absorption intensity of ZnIn2S4/Ti3C2 gradually increased due to the full-spectrum absorption of dark Ti3C2 [23]. Because of the high efficiency of photothermal conversion of Ti3C2 [22], it is possible to activate the catalyst by converting the light energy into heat to promote the surface catalytic reaction [29].  DRS was used to measure the light harvesting capability of the photocatalyst samples. A clear red shift was exhibited for the ZnIn 2 S 4 /Ti 3 C 2 nanocomposites, and their absorption in the visible light region was significantly enhanced compared with pure ZnIn 2 S 4 ( Figure 11). As the Ti 3 C 2 content increased, the absorption intensity of ZnIn 2 S 4 /Ti 3 C 2 gradually increased due to the full-spectrum absorption of dark Ti 3 C 2 [23]. Because of the high efficiency of photothermal conversion of Ti 3 C 2 [22], it is possible to activate the catalyst by converting the light energy into heat to promote the surface catalytic reaction [29].

Photocatalytic Activity
The photocatalytic performance of the samples was measured according to the hydrogen evolution from water under visible light irradiation (>420 nm). As expected, a significant enhancement in the photocatalytic activity for hydrogen evolution was observed upon loading of a small amount of Ti3C2 ( Figure 12). When the mass ratio of Ti3C2 reaches 5% (ZIST-5), the best photocatalytic activity is obtained (2.6 mmol·g −1 ·h −1 ). However, with the further increase in Ti3C2 content, the photocatalytic activity gradually decreased. This may be due to an excess of Ti3C2 causing the composite materials to become too conductive. The result is the depression in charge separation efficiency over the composite [30]. In addition, the presence of Ti3C2 with a large amount of dark will also have a shading effect, which is not conducive to the light absorption of composite materials. In addition, the recyclability of ZIST-5 for photocatalytic hydrogen production was also evaluated.
The cycling experiments show that the yield of hydrogen could still reach 2.4 mmol·g −1 ·h −1 after four cycles (Figure 13a). The XRD spectra show that the crystal structure of the recovered catalyst remains intact except for some weakening of crystallinity, indicating that it has excellent stability. For a comparison, the same proportions of Pt and graphene (5

Photocatalytic Activity
The photocatalytic performance of the samples was measured according to the hydrogen evolution from water under visible light irradiation (>420 nm). As expected, a significant enhancement in the photocatalytic activity for hydrogen evolution was observed upon loading of a small amount of Ti 3 C 2 ( Figure 12). When the mass ratio of Ti 3 C 2 reaches 5% (ZIST-5), the best photocatalytic activity is obtained (2.6 mmol·g −1 ·h −1 ). However, with the further increase in Ti 3 C 2 content, the photocatalytic activity gradually decreased. This may be due to an excess of Ti 3 C 2 causing the composite materials to become too conductive. The result is the depression in charge separation efficiency over the composite [30]. In addition, the presence of Ti 3 C 2 with a large amount of dark will also have a shading effect, which is not conducive to the light absorption of composite materials. In addition, the recyclability of ZIST-5 for photocatalytic hydrogen production was also evaluated. The cycling experiments show that the yield of hydrogen could still reach 2.4 mmol·g −1 ·h −1 after four cycles (Figure 13a). The XRD spectra show that the crystal structure of the recovered catalyst remains intact except for some weakening of crystallinity, indicating that it has excellent stability. For a comparison, the same proportions of Pt and graphene (5 wt.%) were loaded on ZnIn 2 S 4 , respectively. Under the same experimental conditions, ZnIn 2 S 4 /Pt and ZnIn 2 S 4 /graphene showed lower photocatalytic activities of 1.3 mmol·g −1 ·h −1 and 1.0 mmol·g −1 ·h −1 than of ZIST-5, respectively ( Figure 14). These results indicate that ZIST-5 is a promising photocatalyst for photocatalytic hydrogen production.

The Photoluminescence Spectra and Electrochemical Impedance Spectra
In order to further comprehend the role of Ti3C2 in improving the photocatalytic activity of ZnIn2S4, the photoluminescence (PL) spectra and electrochemical impedance spectra (EIS) were recorded. PL emission is generated by the recombination of electrons and holes; therefore, lower PL intensity indicates higher separation efficiency of photogenerated electron-hole pairs [30]. As shown in Figure 15, the PL spectra show that the

The Photoluminescence Spectra and Electrochemical Impedance Spectra
In order to further comprehend the role of Ti 3 C 2 in improving the photocatalytic activity of ZnIn 2 S 4 , the photoluminescence (PL) spectra and electrochemical impedance spectra (EIS) were recorded. PL emission is generated by the recombination of electrons and holes; therefore, lower PL intensity indicates higher separation efficiency of photogenerated electron-hole pairs [30]. As shown in Figure 15, the PL spectra show that the main emission peaks of ZnIn 2 S 4 and ZIST-5 appear at 537 nm, while the intensity of the emission peak of ZIST-5 is much weaker than that of ZnIn 2 S 4 . This suggests the heterogeneous junction formed between Ti 3 C 2 and ZnIn 2 S 4 can enhance the efficiency of photogenerated electron and hole separation. In photocurrent response spectra (Figure 16), ZIST-5 showed a higher photocurrent than ZnIn 2 S 4 and, thus, more efficient separation and transportation of photoinduced charge carriers [31]. In the EIS Nyquist plot, a smaller arc radius indicates lower charge transfer resistance at the electrode surface and higher separation efficiency for electron-hole pairs [32]. In Figure 17, the arc radius of ZIST-5 is a little less than that of pure ZnIn 2 S 4 in the darkness, suggesting that ZIST-5 has more effective separation of photogenerated charges. Under visible light irradiation, the arc radius of ZIST-5 and ZnIn 2 S 4 significantly decreases compared with that of darkness, and ZIST-5 decreases more than that of ZnIn 2 S 4 . These results show that in ZIST-5, Ti 3 C 2 acts as an electron acceptor to capture photoelectrons produced by ZnIn 2 S 4 due to its good electrical conductivity. Moreover, it can increase the photocatalytic activity by effectively inhibiting photoinduced carrier recombination.

The Photocatalytic Mechanism
Based on the findings presented above, we propose the possible mechanism of the photocatalytic reaction for this system as shown in Scheme 1. Upon visible light irradiation, electrons (e − ) are excited from the valence band (VB) to the conduction band (CB) of the ZnIn2S4 semiconductor, creating holes (h + ) in the VB. Electrons are transferred to the Ti3C2 nanosheets and accumulate. These electrons can effectively reduce H2O (or H + ) to produce H2, and the accumulated (h + ) in the VB of ZnIn2S4 react with S 2− /SO3 2− as a regenerative reagent. The presence of Ti3C2 can serve as an excellent electron acceptor and mediator, with effective separation of holes and electrons. This results in a significant improvement in the photocatalytic activity.

The Photocatalytic Mechanism
Based on the findings presented above, we propose the possible mechanism of the photocatalytic reaction for this system as shown in Scheme 1. Upon visible light irradiation, electrons (e − ) are excited from the valence band (VB) to the conduction band (CB) of the ZnIn 2 S 4 semiconductor, creating holes (h + ) in the VB. Electrons are transferred to the Ti 3 C 2 nanosheets and accumulate. These electrons can effectively reduce H 2 O (or H + ) to produce H 2 , and the accumulated (h + ) in the VB of ZnIn 2 S 4 react with S 2− /SO 3 2− as a regenerative reagent. The presence of Ti 3 C 2 can serve as an excellent electron acceptor and mediator, with effective separation of holes and electrons. This results in a significant improvement in the photocatalytic activity.

The Photocatalytic Mechanism
Based on the findings presented above, we propose the possible mechanism of the photocatalytic reaction for this system as shown in Scheme 1. Upon visible light irradiation, electrons (e − ) are excited from the valence band (VB) to the conduction band (CB) of the ZnIn2S4 semiconductor, creating holes (h + ) in the VB. Electrons are transferred to the Ti3C2 nanosheets and accumulate. These electrons can effectively reduce H2O (or H + ) to produce H2, and the accumulated (h + ) in the VB of ZnIn2S4 react with S 2− /SO3 2− as a regenerative reagent. The presence of Ti3C2 can serve as an excellent electron acceptor and mediator, with effective separation of holes and electrons. This results in a significant improvement in the photocatalytic activity.

Synthesis of Samples
All reagents were of analytical grade and used without further purification. Ti 3 AlC 2 powder (>98 wt.% purity) was purchased from Beijing Lianli New Technology Co., Ltd., Beijing, China. By etching the Al layer of Ti 3 AlC 2 with HF solution, Ti 3 C 2 was obtained. In brief, 1 g Ti 3 AlC 2 powder was slowly combined with 30 mL HF solution (content ≥ 40 wt.%, Xilong Chemical Co., Ltd., Shantou, China), and the mixture was stirred at room temperature for 72 h. The suspension was filtered and washed with deionized water several times, until neutral pH was achieved. The obtained Ti 3 C 2 was dried under vacuum at 60 • C for 2 h.

Characterizations
Powder X-ray diffraction (XRD) data were collected using a Rigaku TTRAX III X-ray diffractometer (Rigaku D/max-3B, Tokyo, Japan) with Cu Kα radiation. A field emission scanning electron microscope (SEM) (FEIQuanta200FEG microscope, FEI, Eindhoven, The Netherlands) was used to examine the morphology of the sample. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained on a JEM-2100 microscope (JEOL, Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) was performed on a K-Alpha XPS system (Thermo Fisher Scientific, Waltham, MA, USA) with a monochromatic Al Kα source. A Shimadzu UV-2600 photometer was used to record the UV-vis diffuse reflectance spectra (UV-vis DRS, Shimadzu, Kyoto, Japan), using BaSO 4 as a reflectance standard. A PGSTAT 302N electrochemical analyzer (Metrohm, Herisau, Switzerland) was used to record the photocurrent response spectra and electrochemical impedance spectra (EIS) in a standard three-electrode system. The prepared samples were used as the working electrodes with an active area of ca. 0.25 cm 2 on ITO conductive glass. Pt wire was the counter electrode, a standard calomel electrode was used as the reference electrode, and 0.5 M Na 2 SO 4 aqueous solution was used as the electrolyte. A F-7000 fluorescence spectrophotometer (Hitachi, Tokyo, Japan) was used to record the photoluminescence (PL) spectra of powder samples at an excitation wavelength of 325 nm.

Photocatalytic H 2 Generation
Photocatalytic H 2 evolution experiments were conducted in a closed gas circulation and evacuation system fitted with a Pyrex glass window (LABSOLAR-H2I, Beijing Perfectlight Technology Co., Ltd., Beijing, China). In 100 mL of a mixed aqueous solution containing 0.05 M Na 2 SO 3 (Xilong Scientific Co., Ltd., Shantou, China) and 0.05 M Na 2 S (Xilong Scientific Co., Ltd., Shantou, China), 20 mg of photocatalyst was suspended, and the suspension was irradiated with a 300 W Xe lamp (CEL-HXF300, China Education Au-light Co. Ltd., Beijing, China) equipped with a 420 nm cut-off filter. Using a circulating water bath, the solution was maintained at room temperature during the reaction. The concentration of H 2 was directly detected by an on-line gas chromatograph (GC-9750, Chengdu Fuli Instrument Co., Ltd., Chengdu, China) equipped with a thermal conductivity detector (TCD).

Conclusions
In conclusion, a facile hydrothermal method was employed for the successful preparation of 2D/2D ZnIn 2 S 4 /Ti 3 C 2 composite photocatalysts. The 2D/2D ZnIn 2 S 4 /Ti 3 C 2 heterostructures showed excellent interface activity, ideal photoresponsivity properties, and high photogenerated carrier mobility. Our findings suggest Ti 3 C 2 MXene has a unique synergistic effect on photocatalytic hydrogen production due to its unique two-dimensional structure, fast carrier migration rate, and ideal interfacial effect. The separation of photogenerated carriers and the transfer of photoelectrons to MXene nanosheets were significantly promoted by the incorporation of the heterojunction between ZnIn 2 S 4 and Ti 3 C 2 . As a result, we observed a significant improvement in the photocatalytic activity. This work provides a novel synthetic method for the preparation of 2D MXene composite photocatalytic materials and expands the utility of these materials to energy storage and conversion applications.